The concept of hormesis says that small doses of toxins can actually promote health. Chemical compounds such as resveratrol or sulforaphane cause an increase in cellular stress defense mechanisms. Too much of anything can be toxic; as Paracelsus said, the dose makes the poison, i.e. anything can be poisonous in the right dose, even normally innocuous substances like water.
The concept of hormesis applies to exercise as well. Contrary to what might be expected, exercise reduces levels of oxidative stress. The way it does so is by inducing oxidative stress in the first place; the body reacts by increasing levels of antioxidant enzymes and glutathione, overcomes exercise-induced oxidative stress, and becomes healthier than before.
But too much exercise has the potential to be damaging, since the dose makes the poison. How much is too much? Researchers looked at data from the Copenhagen City Heart Study, in which 1,098 healthy joggers and 3,950 healthy nonjoggers have been prospectively followed up since 2001.
They found that the optimal jogging frequency in terms of mortality was 2 to 3 times a week, or even only 1 or fewer times a week. Joggers in the former category had a hazard ratio of .32, meaning that they were only about 1/3 as likely to die during the study period as sedentary people. The best pace for mortality was slow to moderate.
However, joggers, which in this case we would term runners, who were in the category that the researchers deemed “strenuous”, had a hazard ratio of 1.97, meaning that they were nearly twice as likely to die during the study period as sedentary people.
This must come as a shock to many, since the notion that fitness equals health is a popular one. In reality, the “strenuous” joggers were probably quite fit, yet they died at a much higher rate.
The researchers concluded:
The findings suggest a U-shaped association between all-cause mortality and dose of jogging as calibrated by pace, quantity, and frequency of jogging.
The U-shaped curve is exactly what would be expected from exercise as hormesis. As exercise increases from the sedentary point, mortality drops to a low; then increases as more exercise is added.
It must be said that a degree of statistical uncertainty exists here, as can be seen in the lowest bar on the graph, but the likeliest hazard ratio was approximately 2.0.
I used to be one of those strenuous runners. I did what I was told, that is, exercise more and more so I could be “healthy”, yet I ended up with a case of chronic fatigue that lasted many years, until I figured out how to overcome it.
Although to my knowledge no study like this has been done with weight trainers, it seems entirely possible that one could overdo it. Training 5 or more days a week might be one of those ways of overdoing it. In a recent video lecture, Doug McGuff, author of Body by Science, recounted how he trained in the gym hard for three days a week for many years, and constantly felt like, in his words, “dog crap”. McGuff advocates one day a week in the gym, although I believe that that is a bit too cautious and you can exercise more than that while still retaining good health.